It is understood that water tree is a kind of micro-deterioration of an insulation caused by the simultaneous exposure of a cable insulation to both water and an electric field. Particularly, it has been known that water trees develop by the interaction between water, the void formed inside the insulation, and impurities from outside in the electric field-concentrated region, for example a rough interface or protrusion between an insulation and the semiconductive layer.
The conventional methods to inhibit water trees are as follows; a substitution of steam curing process with gas curing process in order to prevent water or impurities from entering a cable, and to minimize the void and moisture forming in the cable insulation during cable production; triple coextrusion to enhance interfacial smoothness between the semiconductive layer and the insulating layer; and interception of incoming water or impurities such as ions by utilizing a metal shielding layer or water-absorbing tape, or by replacing PVC jacket with PE jacket.
Even if a cable is produced by one of the above methods, water and voids are inevitably formed as a cross-linking by-product during cable production processes and there is no way to intercept water or ions entering from the external environment.
Accordingly, water-tree mediated incidents have been frequently reported and thus there is an urgent need to develop a new insulation material having strong resistance against water-tree generation to provide electrical stability and efficiency.
Attempts made to overcome the above problems are illustrated in the following descriptions:
U.S. Pat. No. 4,206,260 illustrates a method using an inorganic filler such as polycarboxylic ester, fatty acid metal salt, organic isocyanate or silicon compound, starting with the addition of alcohol.
In particular, U.S. Pat. Nos. 4,305,849 and 4,440,671 describe that water tree growth is remarkably retarded by the addition of 0.5-1.0 weight part of polyethylene glycol having a molecular weight of 1,000-20,000.
Although 4,4′-thiobis(2-t-butyl-5-methylphenol) used in the above patents is the representative thermal oxidation inhibitor, which has been widely applied to cross-linked insulation for high-voltage cables, it is a fine powder and has a high melting point of 160-163° C., suggesting that it cannot be melted well at the desired cable processing temperature of 125-135° C. Thus, mixing and dispersion with polyethylene melted in the extruder is decreased. An excess of quantity of 4,4′-thiobis(2-t-butyl-5-methylphenol) (more than 0.3 weight part) results in color change and sweat-out to the surface of a pellet, particularly when the temperature gap between the inside and outside is excess, and especially during summer and winter, during the long-term storage of the cross-linked polyethylene pellet (XLPE Pellet). At this time, unstable extrusion attributed to the dust generated during the transfer process through the extruder hopper is a problem.
If 4,4′-thiobis(2-t-butyl-5-methylphenol) is over-used, it remains locally indispersed in the vulcanization zone at high temperature, indicating that it is not able to provide regular thermal oxidation properties in the cable insulation, and even reduces the electrical insulation properties of the cable by providing a source for the generation and growth of water trees because of the local oxidation defect.
In the meantime, Korean Patent Publication No. 2002-007925 illustrates that the addition of liquid type 2,4-bis(n-octylthiomethyl)-6-methylphenol or 2,4-bis(n-dodecylthiomethyl)-6-methylphenol as an antioxidant might improve scorch resistance and blooming (sweat-out), compared with when the conventional amine or phenol antioxidant is used. However, the over-dose of such antioxidants (0.3 weight part) might also generate voids in the vulcanization zone at high temperature with high pressure.